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Enzymes employ metal ions (e.g. Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn) for catalysing certain reactions.

Let's consider planting a seed. Where will it obtain these metal ions from? Just from water?

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    $\begingroup$ Yes. From the soil. $\endgroup$ – WYSIWYG May 30 '16 at 4:17
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    $\begingroup$ @WYSIWYG then how hydroponics work? (Growth solution seems to only have N, P, K) $\endgroup$ – Sparkler May 30 '16 at 11:23
  • $\begingroup$ Even analytical ultra pure NaCl has so much Ca2+ in it that it would disrupt intracellular processes. Even if it says only N, P and K, there are still sufficient trace metal impurities. $\endgroup$ – Ashafix May 30 '16 at 11:45
  • $\begingroup$ @Ashafix ...which come from the water content..? $\endgroup$ – Sparkler May 30 '16 at 11:59
  • $\begingroup$ Water and nutrients like fertilizer. $\endgroup$ – Ashafix May 30 '16 at 12:14
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Many ions are absorbed by the plants via the action of bacterias. Fe or B for example require chelating action from microbes. P and S need to be oxidised to enter the plant as well. Source: Le sol, la Terre et les Champs - http://www.sangdelaterre.fr/index_135_151.html

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The question is a little bit dubious, because when the OP says "seed" we tend to think about the seed before germination.

If you are talking strictly about the seed, it doesn't absorb from anywhere: the seed normally contains inside it all the macro and micro nutrients necessary for the first days after germination.

Of course, after a while, the growing sporophyte will need more macro and micro nutrients. These nutrients are already dissolved in soil (or in the water, for the aquatic plants), as non-charged compounds or as ions, and are absorbed mainly by the root system (whose cells have several different channels for absorbing them).

When the soil is poor in one of these nutrients, very interesting evolutive strategies may develop to obtain nutrients, as in the "carnivorous" plants. Note: the nutrients are classified in macro and micro not by the size or the weight of the atom, as the name may imply, but by the amount needed.

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  • $\begingroup$ I'd like to add a small correction here. In plants the major micronutrient is glucose, which the plant synthesizes from carbon dioxide and water (mainly)...so you should omit the word 'macro'-nutrients here. $\endgroup$ – Arnb Sep 4 '16 at 6:32

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